Sensor packages

ABSTRACT

A sensor package comprising: a sensor, wherein the sensor comprises a sensing structure formed in a material layer and one or more further material layers arranged to seal the sensing structure to form a hermetically sealed sensor unit; a support structure; one or more springs flexibly fixing the hermetically sealed sensor unit to the support structure; wherein the one or more springs are formed in the same material layer as the sensing structure of the sensor unit; and one or more external package wall(s) encapsulating the sensor unit, the support structure, and the one or more springs, wherein the support structure is fixed to at least one of the package wall(s). The springs decouple mechanical stresses between the sensor unit and the external package wall(s) so as to reduce the long term drift of scale factor and bias.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 16/511,433filed Jul. 15, 2019, which claims priority to Great Britain PatentApplication No. 1811925.5 filed Jul. 20, 2018, the entire contents ofwhich are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to sensor packages, in particular to MEMSsensor packages.

BACKGROUND

Sensors, for example pressure sensors or inertial sensors (such asaccelerometers and gyroscopes) are used in many applications, includinginertial navigation, robotics, avionics, and automobiles. In inertialnavigation applications, such sensors may be found in self-containedsystems known as “inertial measurement units” (IMUs). IMUs typicallycontain a plurality of accelerometers and/or gyroscopes, and provide anestimate of an object's travel parameters such as angular rate,acceleration, altitude, position, and velocity, based on the outputs ofgyroscope(s) and/or accelerometer(s). Each inertial sensor in an IMU isa self-contained package. An IMU typically consists of accelerometersand gyroscopes sensing in all three axes. This is normally part of anInertial Navigation System (INS), which adds computation of velocity andposition using navigation algorithms. At IMU level, the outputs areusually limited to angular rotation and velocity increments with eachsample.

Microelectromechanical systems (MEMS)-based sensors, typicallyfabricated from a single silicon wafer, can be used e.g. to measurepressure or temperature, or linear or angular motion without a fixedpoint of reference. MEMS pressure sensors often work on the principle ofmechanical deformation of a MEMS structure due to fluid pressure. MEMSgyroscopes, or strictly speaking MEMS angular rate sensors, can measureangular rate by observing the response of a vibrating MEMS structure toCoriolis force. MEMS accelerometers can measure linear acceleration byobserving the response of a proof mass suspended on a spring in a MEMSstructure. High performance MEMS inertial sensors are defined by theirbias and scale factor stability.

A MEMS sensor is usually supported on an isolation layer within itspackage. For example, an isolation layer of silicone elastomer may beprovided between the package and the lowermost glass layer of the MEMSsensor. In some examples, the MEMS sensor may be mounted on an isolationlayer including a raft that is connected to the surrounding package viasprings or other damping structures. The isolation layer has two mainfunctions: to provide isolation from unwanted external vibrations; andto absorb mechanical stress due to thermal expansion differences betweenthe MEMS sensor and the surrounding package (typically alumina orceramic).

The stability of the isolation layer that attaches a MEMS inertialsensor to its package is important for high performance, especially whentrying to achieve better than 0.1 mg bias stability. An elastomericisolation layer is usually chosen to have a very low elastic modulus(e.g. silicone) to decouple the MEMS sensor from package stresses.However, such materials suffer from long term creep and ageing effectswhich can therefore alter sensor performance (e.g. bias and scalefactor) by virtue of stress relief over a period of time in service. Itis therefore difficult to achieve good isolation of an inertial sensorfrom package stresses and good long term stability in performance.Similar considerations apply when mounting any MEMS sensor in a package.

There remains a need for improved isolation mounting in sensor packages.

SUMMARY

According to a first aspect of this disclosure, there is provided asensor package comprising: a sensor, wherein the sensor comprises asensing structure formed in a material layer and one or more furthermaterial layers arranged to seal the sensing structure to form ahermetically sealed sensor unit; a support structure; one or moresprings flexibly fixing the hermetically sealed sensor unit to thesupport structure; wherein the one or more springs are formed in thesame material layer as the sensing structure of the sensor unit; and oneor more external package wall(s) encapsulating the sensor unit, thesupport structure, and the one or more springs, wherein the supportstructure is fixed to at least one of the package wall(s).

According to a second aspect of this disclosure, there is provided amethod of manufacturing a sensor package, the method comprising: forminga sensing structure in a material layer; forming one or more springs inthe same material layer as the sensing structure; adding one or morefurther material layers to seal the sensing structure to form ahermetically sealed sensor unit with the one or more springs flexiblyfixing the hermetically sealed sensor unit to a support structure; andfixing the support structure to one or more external package wall(s),the one or more external package wall(s) encapsulating the inertialsensor unit, the support structure, and the one or more springs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art sensor package that uses a full elastomeric diebond.

FIG. 2 shows a sensor package in accordance with an example of thepresent disclosure.

FIG. 3 shows a plan view of a first material layer in accordance with anexample of the present disclosure.

FIG. 4 shows a further plan view of the first material layer, includingsqueeze damping fingers, in accordance with an example of the presentdisclosure.

FIGS. 5a-5k show a process for manufacturing a sensor package inaccordance with an example of the present disclosure.

FIGS. 6a-6k show a process for manufacturing a sensor package inaccordance with another example of the present disclosure.

DETAILED DESCRIPTION

The present disclosure relates to a sensor package and a method ofmanufacturing a sensor package. It will be appreciated that forming oneor more springs in the same material layer as the sensing structure ofthe sensor unit is a completely different approach to sensor isolationthan the conventional way of mounting the hermetically sealed sensorunit to an external package using a different material, e.g. anelastomeric material, as an isolation layer. The springs can decouplemechanical stresses between the sensor unit and the external packagewall(s) so as to reduce the long term drift of scale factor and bias.Furthermore, the effects of temperature sensitivity on bias and scalefactor can also be reduced e.g. for an inertial sensor.

Some non-limiting examples of a sensor package and a method ofmanufacturing a sensor package are described in further detail below.

FIG. 1 shows a prior art inertial sensor package 100 comprising an upperglass layer 102, a silicon sensing layer 104, a lower glass layer 106,an elastomeric die bond layer 110, an alumina substrate 112, and apackage lid 114. The three layers 102, 104 and 106 make up ahermetically sealed inertial sensor unit 108.

The silicon sensing structure in the layer 104 is sensitive to some formof applied force, in this case, acceleration. This silicon sensing layer104 is sandwiched between the upper glass layer 102 and the lower glasslayer 106 to form a hermetically sealed inertial sensor unit 108. Thissealing allows for the silicon sensing structure in the layer 104 tooccupy a space within a controlled environment. The inside of thehermetically sealed inertial sensor unit 108 comprises an atmospherethat is controlled to optimise sensor performance.

The hermetically sealed inertial sensor unit 108 is attached to thealumina substrate 112 by elastomeric die bond layer 110. As shown, theelastomeric die bond layer 110 covers all of the lower glass layer 106of the hermetically sealed inertial sensor unit 108. The elastomeric diebond layer 110 is flexible. This provides good adhesion for thehermetically sealed inertial sensor unit 108 to the alumina substrate112, and further provides good mechanical stability under shock andvibration of the inertial sensor package 100.

However, as described in the background section above, thermal andmechanical stresses across the inertial sensor package 100 can adverselyaffect the performance of inertial sensor unit 108, especially later inthe lifetime of the device.

Package lid 114 allows the hermetically sealed inertial sensor unit 108to be further protected from external environmental influences, such asdirt or direct force application. Typically, this lid is a metal alloyor alumina and is soldered to the substrate 112. The package formed bylid 114 also forms a hermetic seal, and allows for controlling thegaseous environment inside the package. This is done to ensure anoptimal dry gas environment inside the package, and prevents anymoisture ingress, which may negatively affect sensor performance,particularly in a capacitive-type inertial sensor.

FIG. 2 shows a sensor package, e.g. an inertial sensor package 200 inaccordance with an example of the present disclosure. The boundaries ofthe inertial sensor package 200 are defined by three external walls 216,and a substrate 220. The inertial sensor package 200 comprises a siliconsensing structure 204 formed in a silicon material layer 202 fordetecting an applied acceleration. The silicon sensing structure 204 ishermetically sealed into an inertial sensor unit 210, by an upper glasslayer 206 and a lower glass layer 208. The inertial sensor package 200also comprises a support structure 214 formed in the silicon layer 202.The support structure 214 takes the form of a frame surrounding thehermetically sealed inertial sensor unit 210. The support structure 214is attached to the substrate 220, through the intervening glass layer208, by a compliant or fixed mount 218. The inertial sensor unit 210 isdecoupled from the substrate 220, as it is instead suspended from thesupport structure 214 by a plurality of springs 212, formed in thesilicon material layer 202. In this example, the silicon sensingstructure 204 is electrically connected to an external connection 226via flexible wire bonds 224, and through-hole vias 222 in the upperglass layer 206.

As shown, the sensor unit 210 is decoupled from stresses and suddenforces applied to the package 200 in two ways. Primarily, the springs212 that join the ‘floating’ sensor unit 210 to the support structure214 compensate for any such stresses, leaving the sensor unit 210 freefrom such imbalances, decreasing long term drift of scale factor andbias. Furthermore, the effects of temperature sensitivity on bias andscale factor can also be reduced. Secondary to the effects of thesprings 212, the elastomeric mount 218 can also absorb some stresses andshocks to the package 200. In these ways, sensor performance isimproved.

As shown, parts of the support structures 214, the springs 212, and thesensing structure 204 are all made in the same silicon material layer202. This may have significant benefits to the manufacturing process, asstreamlined development can take place, enabling the device to be mostlymanufactured before needing to singulate the parts from a wafer. Thisbatch processing of the devices may both increase throughput anddecrease cost of manufacture. By manufacturing the devices this way, thesensor unit 210 can be conveniently decoupled from the supportstructures 214 by etching out the springs 212 during the same processthat is used to etch out the sensing structure 204.

An electrical connection is made to the sensing structure 204 with aconductive e.g. metal path passing down the through-hole vias 222. Thisconnection is then carried down to the substrate 220, via the flexiblewire bonds 224, to meet the external connections 226 which allow anelectrical connection to be made from the outside of the sensor package200 directly to the sensing structure 204. The flexible wire bonds 224are flexible enough to withstand any stresses or gradients within thepackage, for example flexing of the springs 212.

The upper 206 and lower 208 glass layers form a hermetic seal around thesensor unit 210. The environment within the sensor unit 210 can becontrolled at the time of sealing, and in this example, the sensor isfilled with dry Nitrogen at atmospheric pressure. This controlledenvironment enables tuning of the damping factor within the hermeticallysealed sensor unit 210.

The external walls 216 form a hermetic seal around the supportstructures 214 and the sensor unit 210. The environment within this areacan be controlled upon sealing as well. The control of this environmentenables tuning of the damping factor of the squeeze film damping fingers(not shown in FIG. 2), which is explained in more detail below withreference to FIG. 4.

FIG. 3 shows a plan view of the first material layer in accordance withan example of the present disclosure. The first material layer is asilicon layer 300. Silicon layer 300 is made from a single sheet ofcrystalline silicon. The silicon layer 300 comprises a support structure302, taking the shape of an outer frame, a plurality of springs 304, anda sensing structure 306. There is no residual glass in the spring area.The support structure 302 is a frame surrounding the sensing structure306.

As shown, the sensing structure 306 is suspended from the support frame302 by the springs 304. The springs 304 decouple the sensing structure306 from the mechanical and other stresses experienced by the inertialsensor package 200, whilst having a spring constant such that inertialmovement is still transferred to the sensing structure 306. The resonantfrequency of the springs 304 is between 1-5 kHz, for example 2 kHz. Thelength and width of the springs 304 is designed in order to select anoptimal resonant frequency. The springs 304 are serpentine in shape, andhave a number of serpentine turns, for example between 1-10 turns.

The manufacture of the support structure 302, springs 304 and sensingstructure 306 all in a single material layer allows for a streamlinedmanufacturing process, saving both cost and time.

FIG. 4 shows a further plan view of the first material layer 400,including optional squeeze film damping fingers 406, in accordance withan example of the present disclosure. Shown in FIG. 4 is a supportstructure 402, a sensing structure 408, springs 404, and squeeze filmdamping fingers 406.

As in FIG. 3, the springs 404 suspend the sensing structure 408 from thesupport structure 402. The addition of the squeeze film damping fingers406 assists in the decoupling of mechanical and other stresses betweenthe inertial sensor package and support structure, and the sensingstructure 408. The squeeze film damping fingers 406 help to provide nearcritical damping to the inertial sensor package, preventing the sensingstructure 408 from being damaged. The squeeze film damping fingers 406do this by limiting the range of movement of the sensing structure 408with respect to the support structure 402.

The damping effect of the squeeze film damping fingers 406 can be tunedby altering the composition of the inertial sensor package environment,for example by filling it with dry Nitrogen, Neon or Argon atatmospheric pressure. The damping effect can also be tuned by adjustingthe number of fingers, the lengths of the fingers, and the size of thegaps between the fingers.

FIGS. 5a-5k shows a process for manufacturing a sensor package inaccordance with an example of the present disclosure.

FIG. 5a shows the first step of pre-cavitating a layer of glass 502 witha wet etch. The etch is defined by a mask, and the glass layer 502 isonly etched in the region which a moving sensing structure will lateroccupy. The depth of the etch is typically around 30 μm.

FIG. 5b shows the next step of anodically bonding a silicon wafer 504 tothe glass layer 502.

FIG. 5c shows the next step of creating through-hole vias 506. This istypically done by first applying a photomask (not shown), and powderblasting the glass layer 502 in order to create the through-hole vias506. The through-hole vias 506 go through to the silicon layer 504,inside the pre-cavitated area of the glass layer 502.

FIG. 5d shows the next step of depositing a metal tracking layer 508onto the glass layer 502, forming a uniform thin layer 508 coating theglass layer 502, and the inside surfaces of the through-hole vias 506 inthe process. Alternatively, the metal tracking layer 508 may fill thethrough-hole vias 506. This allows electrical connections to be made tothe silicon layer 504, in order to connect a sensing structure made inthe silicon layer 504 with an external package. The metal tracking layer508 is typically deposited and then patterned by photo-lithography.

FIG. 5e shows the next step of performing an isotropic wet etch on theglass layer 502, in the regions 510 where the springs will be formed, inorder to suspend the sensing structure later formed in the silicon layer504. A photomask (not shown) is used to protect the other areas from thewet etch. This exposes the underlying silicon layer 504.

FIG. 5f shows the next step of performing a Deep Reactive Ion Etch(DRIE) on the underlying silicon layer 504 from the bottom. A standardphoto mask (not shown) is used to define the etched regions of thesilicon layer 504. In this step, a sensing structure 512 is etched fromthe silicon layer 504. This etch also defines a plurality of serpentinesprings 514 suspending the sensing structure 512 from the newly definedsupport structure 516.

Next, a lower glass layer 518 is pre-cavitated in moving regions of thesensing structure 512 in the silicon layer 504. As shown in FIG. 5g ,the lower glass layer 518 is then anodically bonded to the silicon layer504, forming a hermetically sealed sensor unit 520 containing thesensing structure 512. The hermetically sealed sensor unit 520 isback-filled with a gas, typically dry Nitrogen, Argon or Neon atatmospheric pressure. This ensures near critical damping of the sensingstructure 512.

FIG. 5h shows the next step of performing an isotropic wet etch on thelower glass layer 518 to the depth of the silicon layer 504. This etchis defined by a photomask (not shown), and leaves only the springs 514in the regions 510 of FIG. 5(e). This also releases the hermeticallysealed sensor unit 520 from the support structure 516, leaving itsuspended by the springs 514. This decouples the hermetically sealedsensor unit 520 from any large shocks or stresses experienced by thesupport structure 516, as they will be absorbed by the springs 514instead. Furthermore, it will be seen that the sensing structure 512 ishermetically isolated from the springs 514 by the anodically bondedglass layers 502, 518, forming the hermetically sealed sensor unit 520.The device may be singulated from the wafer after this stage too. Thisallows for streamlining of production of the devices, as the devices arealmost fully formed before they are singulated from the wafer.

FIG. 5i shows the next step of bonding the support structures 516 to asubstrate 524 (typically made from alumina or ceramic), via one or moreelastomeric mounts 522 (for instance). In this way, the hermeticallysealed sensor unit 520 is decoupled from any stresses or shocksexperienced by the substrate 524 via the springs 514 as well as theelastomeric mounts 522.

FIG. 5j shows the next step of adding flexible wire bonds 526 to thedevice, thereby attaching the metal tracking layer 508 on thehermetically sealed sensor unit 520 to the (relatively) fixed supportstructures 516. Furthermore, flexible wire bonds 526 are also added fromthe support structure 516 to the substrate 524. The wire bonds typicallyhave a diameter of 25 μm. An external electrical connection 525 is alsoadded through the substrate layer 524.

FIG. 5k shows the final step of adding a metal lid 528, hermeticallysealing the internal gas volume of the package. This is typically doneusing solder sealing (at ˜300° C.), securing the lid 528 to thesubstrate 524. The internal gas volume of the package is controlled tooptimise the decoupling between the hermetically sealed sensor unit 520and the support structures 516, and typically comprises Argon, Neon ordry Nitrogen at atmospheric pressure—e.g. to optimise squeeze filmdamping.

FIGS. 6a-6k show a process for manufacturing a sensor package inaccordance with another example of the present disclosure. Themanufacturing process shown in FIGS. 6a -6 ak is similar to that shownin FIGS. 5a-5k , and will be described below with reference to FIGS.5a-5k where appropriate.

The manufacturing process shown in FIGS. 6a-6c is the same as that shownin FIGS. 5a -5 c.

FIG. 6d shows the next step of performing an isotropic wet etch on theglass layer 602, in the regions 610 where the springs will be formed, inorder to suspend the sensing structure. A photomask (not shown) is usedto protect the other areas from the wet etch. This exposes theunderlying silicon layer 604.

FIG. 6e shows the next step of performing a DRIE on the underlyingsilicon layer 604 from the bottom. A standard photo mask (not shown) isused to define the etched regions of the silicon layer 604. In thisstep, a sensing structure 612 is etched from the silicon layer 604. Thisetch also defines the serpentine springs 614 suspending the sensingstructure 612 from newly defined support structure 616.

FIG. 6f shows the next step of depositing a metal tracking layer 608onto the silicon layer 604 and the glass layer 602, forming a uniformthin layer 608 coating the glass layer 602, and the inside surfaces ofthe through-hole vias 606 in the process. Alternatively, the metaltracking layer 608 may fill the through-hole vias 606. This allowselectrical connections to be made to the silicon layer 604, in order toconnect a sensing structure made in the silicon layer 604 with anexternal package. This step also defines metal tracking down the facesof the isotropically etched glass layer 602, and across the surface ofthe springs 614. This provides an electrically conductive path from thesensing structure 612 and the through-hole vias 606, to the edge of theglass layer 602 where the support structure 616 surrounds the sensingstructure 612. The metal tracking layer 608 is typically deposited, andthen patterned by photo-lithography.

Next, a lower glass layer 618 is pre-cavitated in moving regions of thesensing structure 612 in the silicon layer 604. As shown in FIG. 6g ,the lower glass layer 618 is then anodically bonded to the silicon layer604, forming a hermetically sealed sensor unit 620 containing thesensing structure 612. The hermetically sealed sensor unit 620 isback-filled with a gas, typically dry Nitrogen, Argon or Neon atatmospheric pressure. This ensures near critical damping of the sensingstructure 612.

FIG. 6h shows the next step of performing an isotropic wet etch on thelower glass layer 618 to the depth of the silicon layer 604. This etchis defined by a photomask (not shown), and leaves only the springs 614and the corresponding metal tracking in the regions 610 of FIG. 6(d).This also releases the hermetically sealed sensor unit 620 from thesupport structure 616, leaving it suspended by the springs 614. Thisdecouples the hermetically sealed sensor unit 620 from any large shocksor stresses experienced by the support structure 616, as they will beabsorbed by the springs 614 instead. Furthermore, it will be seen thatthe sensing structure 612 is hermetically isolated from the springs 614by the anodically bonded glass layers 602, 618, forming the hermeticallysealed sensor unit 620. The device may be singulated from the waferafter this stage too. This allows for streamlining of production of thedevices, as the devices are almost fully formed before they aresingulated from the wafer.

FIG. 6i shows the next step of bonding the support structure 616 to asubstrate 624 (typically made from alumina or ceramic), via one or moreelastomeric mounts 622 (for instance). In this way, the hermeticallysealed sensor unit 620 is decoupled from any stresses or shocksexperienced by the substrate 624 via the springs 614 as well as theelastomeric mounts 622.

FIG. 6j shows the next step of adding a flexible wire bond 626 to thedevice, attaching the metal tracking layer 608 at the support structure616 to the substrate layer 624. The wire bonds typically have a diameterof 25 μm. An external electrical connection 625 is also added throughthe substrate layer 524. As previously mentioned, this allows for anexternal electrical connection to be made to the sensing structure 612,but in this example across the electrically conductive paths carried bythe springs 614 between the flexible wire bond 626 and the sensor unit620.

FIG. 6k shows the final step of adding a metal lid 628, hermeticallysealing the internal gas volume of the package. This is typically doneusing solder sealing (at ˜300° C.), securing the lid 628 to thesubstrate 624. The internal gas volume of the package is controlled tooptimise the decoupling between the hermetically sealed sensor unit 620and the support structure 616, and typically comprises Argon, Neon ordry Nitrogen at atmospheric pressure—e.g. to optimise squeeze filmdamping.

It will be appreciated that forming one or more springs in the samematerial layer as the sensing structure provides for ease of manufacturewhile also decoupling mechanical and thermal stresses between the sensorunit and the external package wall(s). More generally, some examples ofa sensor package and a method of manufacturing a sensor packageaccording to the present disclosure are provided below.

According to one or more examples of the present disclosure, the one ormore springs may have a serpentine form. The geometrical form of thesprings may be designed to provide a predefined spring compliance orstiffness. In at least some examples, the one or more springs areconfigured to provide a spring resonance ≥1 kHz and preferably in therange of 1-5 kHz. This has been found by the inventors to give enoughcompliance without compromising sensor performance at lower frequency.

According to one or more examples of the present disclosure, the one ormore springs preferably comprises a plurality of springs. The springsmay be arranged around the sensor unit. For example, the sensor unit maybe arranged centrally within the support structure and the springs mayextend in multiple directions between the sensor unit and the supportstructure. The support structure may be in the same plane as the sensorunit or in a different plane, above and/or below the sensor unit. In oneor more examples, the sensor unit may be suspended by the springs fixingthe sensor unit to the support structure.

According to one or more examples of the present disclosure, thematerial layer in which the sensing structure is formed comprisessilicon. The one or more springs may therefore be formed in the samesilicon layer as the sensing structure of the sensor unit. The siliconsprings can be shaped and/or dimensioned to give radial compliance toallow for stress relief between the sensor unit and the supportstructure. The silicon springs may conveniently be etched out during thesame process that is used to etch out the sensing structure. Forexample, the one or more springs may be formed by etching a serpentineform in the silicon material layer.

According to one or more examples of the present disclosure, thehermetically sealed sensor unit comprises a glass layer, a siliconmaterial layer comprising the sensing structure, and a further glasslayer. Such a material structure is known as a silicon-on-glass (SOG)structure. The one or more further material layers arranged to seal thesensing structure to form a hermetically sealed sensor unit maytherefore be glass layer(s).

According to one or more examples of the present disclosure, thehermetically sealed sensor unit comprises a silicon layer, a siliconmaterial layer comprising the sensing structure, and a further siliconlayer. The one or more further material layers arranged to seal thesensing structure to form a hermetically sealed sensor unit maytherefore be silicon layer(s).

According to one or more examples of the present disclosure, the supportstructure is formed in the same material layer as the sensing structureof the sensor unit and the spring(s). In such examples the supportstructure is in the same plane as the material layer. This means thatthe support structure may be conveniently decoupled from the sensingstructure by etching out the spring(s) during the same process that isused to etch out the sensing structure. The support structure maytherefore be formed from silicon, the same as the sensing structure.

According to one or more examples of the present disclosure, the supportstructure is a frame. The frame may surround the hermetically sealedsensor unit. As mentioned above, a plurality of the springs may extendbetween the sensor unit and the frame e.g. suspending the sensor unitcentrally within the frame.

According to one or more examples of the present disclosure, the supportstructure is fixed to at least one external package wall via a compliant(e.g. elastomeric) mount. Such an elastomeric mount may provide a degreeof compliance, but it will be appreciated that the main decouplingbetween the sensor unit and the support structure is through the one ormore springs. The compliant mount may require much less elastomericmaterial than the conventional elastomeric isolation layer used in priorart sensor packages.

According to one or more alternative examples of the present disclosure,the support structure is fixed to at least one external package wall viaa rigid mount. It will be appreciated that a rigid mount may be used asthe sensor unit is already decoupled from the support structure throughthe one or more springs. The rigid mount may comprise an adhesive e.g.epoxy bond or a metal solder joint.

According to one or more examples of the present disclosure, the sensorpackage further comprises a squeeze film damping structure arrangedbetween the hermetically sealed sensor unit and the support structure.Such a damping structure comprises one or more gaps that are sized so asto provide a squeeze film damping effect in the gaseous atmospherewithin the package, as is known in the art. For example, the squeezefilm damping structure may comprise a plurality of interdigitateddamping fingers. The plurality of interdigitated damping fingers may bearranged in one or more sets, for example multiple sets arranged aroundthe sensor unit.

According to one or more examples of the present disclosure, the squeezefilm damping structure is formed in the same material layer as thesensing structure of the sensor unit and the spring(s). This means thatthe squeeze film damping structure may conveniently be formed during thesame process that is used to etch out the sensing structure and thespring(s). The squeeze film damping structure (e.g. interdigitateddamping fingers) may therefore be formed from silicon, the same as thesensing structure.

According to one or more examples of the present disclosure, thehermetically sealed sensor unit is evacuated. According to one or morealternative examples of the present disclosure, the hermetically sealedsensor unit comprises a first gaseous environment e.g. comprising one ormore of Argon, Neon or dry Nitrogen. The first gaseous environment maybe at a pressure below atmospheric pressure, e.g. partially evacuated.Alternatively, the first gaseous environment may be at a pressure aboveatmospheric pressure. This elevated pressure may give a higher dampingfactor.

According to one or more examples of the present disclosure, the sensorpackage comprises a second gaseous environment outside the hermeticallysealed sensor unit, e.g. made up of one or more of Argon, Neon or dryNitrogen. The second gaseous environment may be at atmospheric pressure.In examples wherein a squeeze film damping structure is arranged betweenthe hermetically sealed sensor unit and the support structure, thesecond gaseous environment may be chosen to provide the desired squeezefilm damping effect.

According to one or more examples of the present disclosure, the sensorpackage further comprises flexible wire bonds electrically connectingthe sensor unit to at least one of the external package wall(s). Thehermetically sealed sensor unit may further comprise one or morethrough-hole vias for electrical connection to the sensing structure.This means that direct wire bonds may pass down the through-hole vias toprovide for electrical connection of the sensing structure.

According to one or more examples of the present disclosure, thehermetically sealed sensor unit is electrically connected to at leastone of the external package wall(s) by an electrically conductive pathcarried by the one or more springs. For example, conductive (e.g. metal)tracking may be carried by the one or more springs. The hermeticallysealed sensor unit may further comprise one or more through-hole viasfor electrical connection to the sensing structure. This means thatdirect wire bonds may pass down the through-hole vias to provide forelectrical connection of the sensing structure. This connection can thenbe linked to the electrically conductive path carried by the one or moresprings in order to provide an electrical connection from at least oneof the external package walls to the sensing structure.

According to one or more examples of the present disclosure, the sensoris a MEMS sensor.

According to one or more examples of the present disclosure, the sensoris a pressure sensor. According to one or more other examples of thepresent disclosure, the sensor is an inertial sensor. It follows thatthe hermetically sealed sensor unit may be a hermetically sealedinertial sensor unit.

According to one or more examples of the present disclosure, theinertial sensor is a gyroscope. The sensing structure may comprise aproof mass in the form of a disc or ring. The gyroscope may be avibrating structure gyroscope.

According to one or more examples of the present disclosure, theinertial sensor is an accelerometer. The sensing structure may comprisea fixed substrate and a proof mass mounted to the fixed substrate byflexible support legs.

According to one or more further examples of the present disclosure, theaccelerometer is one of the following: a capacitive accelerometer, aninductive accelerometer, or a piezoelectric accelerometer. In at leastsome examples, the capacitive accelerometer comprises: a fixed substrateand a proof mass mounted to the fixed substrate by flexible support legsfor in-plane movement along a sensing axis in response to an appliedacceleration; the proof mass comprising a plurality of sets of moveableelectrode fingers extending substantially perpendicular to the sensingaxis and spaced apart along the sensing axis; at least two pairs offixed capacitive electrodes, wherein a first pair of the fixedcapacitive electrodes comprises a first fixed electrode and a fourthfixed electrode, and a second pair of the fixed capacitive electrodescomprises a second fixed electrode and a third fixed electrode, andwherein each fixed capacitive electrode comprises a set of fixedcapacitive electrode fingers extending substantially perpendicular tothe sensing axis and spaced apart along the sensing axis; wherein thesets of fingers of the first and third fixed electrodes are arranged tointerdigitate with the sets of moveable electrode fingers with a firstoffset in one direction along the sensing axis from a median linebetween adjacent fixed fingers, and the sets of fingers of the secondand fourth fixed electrodes are arranged to interdigitate with the setsof moveable electrode fingers with a second offset in the oppositedirection along the sensing axis from a median line between adjacentfixed fingers.

In one or more examples, the method may further comprise: forming thesupport structure in the same material layer as the sensing structure ofthe inertial sensor unit and the one or more springs. As is mentionedabove, this is advantageous as the support structure, spring(s) andsensing structure may all be formed from the same material layer by acommon manufacturing process such as DRIE.

In one or more examples, the method may further comprise: connectingflexible wire bonds between the sensor unit and at least one of theexternal package wall(s).

In one or more examples, the method may further comprise: forming anelectrically conductive path across the one or more springs. Theelectrically conductive path may be formed such that the hermeticallysealed sensor unit is electrically connected to at least one of theexternal package wall(s). For example, the method may further comprise:adding conductive (e.g. metal) tracking to a surface of the one or moresprings. The electrically conductive path may be used to take a signalfrom the sensing structure to the outer frame.

In one or more examples, the method may further comprise: forming asqueeze film damping structure between the hermetically sealed inertialsensor unit and the support structure. Preferably the squeeze filmdamping structure is formed in the same material layer as the sensingstructure of the inertial sensor unit and the spring(s).

1. A method of manufacturing a sensor package, the method comprising:forming a sensing structure in a material layer; forming one or moresprings in the same material layer as the sensing structure; adding oneor more further material layers to seal the sensing structure to form ahermetically sealed sensor unit with the one or more springs flexiblyfixing the hermetically sealed sensor unit to a support structure; andfixing the support structure to one or more external package wall(s),the one or more external package wall(s) encapsulating the sensor unit,the support structure, and the one or more springs.
 2. The method ofclaim 1, further comprising: forming the support structure in the samematerial layer as the sensing structure of the sensor unit and the oneor more springs.
 3. The method of claim 2, further comprising: forming asqueeze film damping structure between the hermetically sealed inertialsensor unit and the support structure.
 4. The method of claim 3, furthercomprising: forming the squeeze film damping structure in the samematerial layer as the sensing structure of the sensor unit and thespring(s).
 5. The method of claim 4, further, comprising: forming anelectrically conductive path across the one or more springs such thatthe sensor unit is electrically connected to at least one of theexternal package wall(s).
 6. The method of any claim 1, further,comprising: forming an electrically conductive path across the one ormore springs such that the sensor unit is electrically connected to atleast one of the external package wall(s).
 7. The method of any claim 2,further, comprising: forming an electrically conductive path across theone or more springs such that the sensor unit is electrically connectedto at least one of the external package wall(s).
 8. The method of anyclaim 3, further, comprising: forming an electrically conductive pathacross the one or more springs such that the sensor unit is electricallyconnected to at least one of the external package wall(s).